see Global warming/temp for a proposed change in the layout of this entry
Global warming is the increase over time of the average temperature of Earth's atmosphere and oceans. In the twentieth century land-based thermometers have recorded such a warming from the 1880s to about 1940, followed by cooling from 1940 to 1975, and another period of warming from 1975 to present (see graph (http://www.grida.no/climate/ipcc_tar/wg1/figspm-1.htm)). The fluctuation in surface temperature is acknowledged by all scientists although the causes remain controversial -- see 'Historical temperature record', 'Urban heat islands' and 'Evidence against a warming period' below.
The climate system varies both through natural, "internal" processes as well as in response to variations in "external forcing" from both human and non-human causes, including changes in solar activity and volcanic emissions as well as greenhouse gases.
The theory that human greenhouse gas emissions are connected with the observed heating of the Earth's atmosphere in the 20th century has steadily gained adherents in the scientific community within the past 15 years, to the extent that many scientific bodies around the world have strongly endorsed it. Even climate researcher Stephen Schneider, who thought in the 1970s that human industrial emissions might lead to global cooling, has become a strong proponent of the global warming hypothesis.
Automobile exhausts, coal-burning power plants, factory smokestacks, and other waste vents of the industrial age now pump six billion tons of carbon dioxide and other greenhouse gases into the earth's atmosphere each year. They are called greenhouse gases because they trap radiant energy from the sun that would otherwise be re-radiated back into space. (The fact that a natural greenhouse effect occurs is well-known and is not at issue in the debate over global warming. Without it, temperatures would drop by approximately 30°C, the oceans would freeze and life as we know it would be impossible.) What climatologists are concerned about, rather, is that increased levels of greenhouse gases in the atmosphere might cause more heat to be trapped. Concentrations of greenhouse gases in the atmosphere are currently at approximately 25% above pre-industrial values. This is considerably higher than at any time during the last 420,000 years, the period for which reliable data exists, from ice cores. From less direct geological evidence it is believed that values this high were last attained 40 million years ago.
Various theories have been proposed to link terrestrial temperature variations to solar variations. The meteorological community has responded with skepticism, in part because theories of this nature have come and gone over the course of the 20th century. The theories have usually been one of three types:
Although often correlations are presented and the mechanism is speculative. Many of these effects have fared badly over time, and in a paper "Solar activity and terrestrial climate: an analysis of some purported correlations" (J. Atmos. and Solar-Terr. Phy., 2003 p801-812) Peter Laut demonstrates problems with some of the most popular, notably those by Svensmark and by Lassen (below).
In 1991, Knud Lassen of the Danish Meteorological Institute in Copenhagen and his colleague Eigil Friis-Christensen found a strong correlation between the length of the solar cycle and temperature changes throughout the northern hemisphere. Initially, they used sunspot and temperature measurements from 1861 to 1989, but later found that climate records dating back four centuries supported their findings. This relationship appeared to account for nearly 80 per cent of the measured temperature changes over this period (see graph (http://solar-center.stanford.edu/images/solactivity.jpg)). Sallie Baliunas, an astronomer at the Harvard-Smithsonian Center for Astrophysics, has been among the supporters of the theory that changes in the sun "can account for major climate changes on Earth for the past 300 years, including part of the recent surge of global warming."  (http://www.news.harvard.edu/gazette/1997/11.06/BrighteningSuni)
On May 6, 2000, however, New Scientist magazine reported that Lassen and astrophysicist Peter Thejll had updated Lassen's 1991 research and found that while the solar cycle still accounts for about half the temperature rise since 1900, it fails to explain a rise of 0.4 °C since 1980. "The curves diverge after 1980," Thejll said, "and it's a startlingly large deviation. Something else is acting on the climate. ... It has the fingerprints of the greenhouse effect." (http://archive.newscientist.com/secure/article/article.jsp?rp=1&id=mg16622370.800)
Later that same year, Peter Stott and other researchers at the Hadley Centre in the United Kingdom published a paper in which they reported on the most comprehensive model simulations to date of the climate of the 20th century. Their study looked at both natural forcing agents (solar variations and volcanic emissions) as well as anthropogenic forcing (greenhouse gases and sulphate aerosols). Like Lassen and Thejll, they found that the natural factors accounted for gradual warming to about 1960 followed by a return to late 19th-century temperatures, consistent with the gradual change in solar forcing throughout the 20th century and volcanic activity during the past few decades. These factors alone, however, could not account for the warming in recent decades. Similarly, anthropogenic forcing alone was insufficient to explain the 1910-1945 warming, but was necessary to simulate the warming since 1976. Stott's team found that combining all of these factors enabled them to closely simulate global temperature changes throughout the 20th century. They predicted that continued greenhouse gas emissions would cause additional future temperature increases "at a rate similar to that observed in recent decades." (http://www.sciencemag.org:80/cgi/content/full/290/5499/2133) A graphical representation (http://www.grida.no/climate/ipcc_tar/wg1/fig12-7.htm) of the relationship between natural and anthropogenic factors contributing to climate change appears in "Climate Change 2001: The Scientific Basis," a report by the Intergovernmental Panel on Climate Change (IPCC).  (http://www.grida.no/climate/ipcc_tar/wg1/index.htm)
Over the past 20,000 years the dominant temperature signal has been the end of the last ice age, approximately 12,000 years ago. (http://www.grida.no/climate/ipcc_tar/wg1/073.htm) Since then the temperature has been quite stable, though with various fluctuations, e.g. Medieval Warm Period or Little Ice Age.
According to the IPCC, Earth has seen a significant increase in average global surface temperature over the last 150 years.
The main evidence for global warming comes from thermometer measurements from land stations and ships  (http://www.met-office.gov.uk/research/hadleycentre/pubs/talks/sld011), which independently show much the same warming since 1860. The data from these stations, particularly those located outside of the United States, show an average surface temperature increase of 0.4 to 0.8 Celsius degrees during the 20th century. Most of the warming occurred during two periods: 1910 to 1945 and 1976 to 2000. (Source: Intergovernmental Panel on Climate Change, the IPCC).
For example, satellite data shows a 10% decrease of snow cover since the late 1960s, and the Northern Hemisphere spring and summer sea-ice extent has decreased by about 10% to 15% since the 1950s and there has been a widespread retreat of mountain glaciers in non-polar regions throughout the 20th century. (Source: IPCC).
Weather balloons and satellites measuring the temperature of the atmosphere above the earth's surface show a smaller warming trend than the trend at the earth's surface (sadly this no longer appears to be true. See the historical temperature record page for details, but the MSU trend is now-this-month at 0.074 oC/decade). Nevertheless, a National Academy of Sciences panel that reviewed this apparent discrepancy concluded that "the warming trend in global-mean surface temperature observations during the past 20 years is undoubtedly real and is substantially greater than the average rate of warming during the twentieth century. The disparity between surface and upper air trends in no way invalidates the conclusion that surface temperature has been rising." (http://books.nap.edu/books/0309068916/html/2#pagetop) (http://www.msnbc.com/news/356875.asp?cp1=1)
Satellite measurements of the troposphere from 1979 to 2002, however, show a warming trend of only 0.04 oC/decade (see graph (http://www.ghcc.msfc.nasa.gov/MSU/msusci)). These satellite data are controversial and incorporate a number of corrections for, amongst other effects, orbital drift of the satellites  (http://www.ghcc.msfc.nasa.gov/MSU/hl_measuretemp.htm). Other analyses of the same data produce different trends. The same page  (http://www.ghcc.msfc.nasa.gov/MSU/msusci) shows a decline in stratospheric temperatures, interspersed by "noise" from volcanic erruptions. This is what is expected from Global Warming theory: the troposphere should warm, whilst the stratosphere should cool. However, this simple picture is complicated by ozone depletion, which also causes a cooling of the stratosphere.
Climate simulations (vague) do not unambiguously attribute the warming that occurred from 1910 to 1945 to either variation in solar radiation (internal and natural forcing) or to anthropogenic forcing (see climate change). The models (which sort?) are used to estimate the relative importance of the various factors mentioned above. Most models show that warming occurring from 1975 to 2000 needs anthropogenic greenhouse gas emissions to be explained.
The IPCC (see below), a United Nations science and public policy organization, published a report saying that scientists believe that anthropogenic greenhouse gases "play an important role in global warming."
This conclusion depends on the accuracy of the models used and on the correct estimation of the external factors. The majority of scientists agree that important climate features are incorrectly accounted for by the climate models but don't think that better models would change the conclusion. (Source: IPCC)
Critics point out that there are flaws in the models and external factors not taken into consideration that could change the conclusion above (which are these?). Some critics say that the climate simulations are unable to model the cooling effects of the particles, fitting the water vapor feedback, and handling clouds (source all this). Critics also point out that the sun may have a share of responsibility for the observed global warming greater than now thought by the majority of the scientific community. Some indirect solar effects may be very important and are not accounted for by the models. So, they argue, the share of global warming caused by anthropogenic greenhouse gases may be lower than thought. (Source: The Skeptical Environmentalist)
This is a matter of considerable controversy, with environmentalist groups typically emphasizing the possible dangers and groups close to industry questioning the climate models and consequences of global warming - and funding scientists to do so.
Due to potential effects on human health and economy due to the impact on the environment, global warming is the cause of great concern. Some important environmental changes have been observed and linked to global warming.
The examples of secondary evidence cited above (lessened snow cover, rising sea levels, weather changes) are examples of consequences of global warming that may influence not only human activities but also the ecosystems. Increasing global temperature means that ecosystems may change; some species may be forced out of their habitats (possibly to extinction) because of changing conditions, while others may spread. Few of the terrestrial ecoregions on Earth could expect to be unaffected.
Another cause of great concern is sea level rise. Sea levels are rising 1 to 2 centimetres (around half an inch) per decade, and some small countries in the Pacific Ocean are expressing concerns that if this rise in sea level continues, they soon will be entirely under water. Global warming causes the sea level to rise mainly because sea water expands as it warms, but some scientists are concerned that in the future, the polar ice caps[?] and glaciers may melt. As a consequence, the sea level could rise several metres. At the moment, scientists are not expecting any major ice melting in the next 100 years. (Sources: IPCC for the data and the mass media for the general perception that climate change is important.) Some researchers have found a negative correlation between sea level rise and average global temperature; water evaporates more quickly than it expands. (Source: Science and Environmental Policy Project)
Global warming can also have other, less obvious effects. The North Atlantic drift, for instance, is driven by temperature changes. It seems as though it is diminishing as the climate grows warmer, and this means that areas like Scandinavia and Britain that are warmed by the drift might face a colder climate in spite of the general global warming. It is now feared that Global Warming may be able to trigger the type of abrupt massive temperature shifts which bracketed the Younger Dryas period.
However, global warming can also have positive effects, since higher temperatures and higher CO2 concentrations improve the ecosystems' productivity. Satellite data shows that the productivity of the Northern Hemisphere has increased since 1982. On the other hand, an increase in the total amount of biomass produced is not necessarily all good, since biodiversity can still decrease even though a small number of species are flourishing. Similarly, from the human economic viewpoint, an increase in total biomass but a decrease in crop harvests would be a net disadvantage. In addition, IPCC models predict that higher CO2 concentrations would only spur growth of flora up to a point; after that, though greenhouse effects and warming would continue there would be no compensatory increase in growth.
Since it is such an important issue, governments need predictions of future trends in global change so they can take political decisions to avoid undesired impacts. Global warming is being studied by the Intergovernmental Panel on Climate Change (IPCC). In its last report, the IPCC made some predictions about future climate change. These predictions are the basis for current political and scientific discussion.
IPCC predictions are based on the same models used to establish the importance of the different factors in global warming. These models need data about anthropogenic emissions of greenhouse gases and aerosols. These data are predicted from economic models[?] based on 35 different scenarios. Scenarios go from pessimistic to optimistic, and predictions of global warming depend on the kind of scenario considered. None of these scenarios consider any kind of measures to avoid global warming.
Nevertheless, proponents of the IPCC assessment say that the current climate models are good in predicting surface temperatures and that this is significant. They furthermore argue that it is surface temperatures that will have the greatest and most direct effect on the environment, agriculture and the stability of polar ice.
The IPCC says that it has corrected the land station data to account for the urban heat island effect. To do: find and summarize their correction technique.
In its last report, IPCC stated that average surface temperature is projected to increase by 1.4 to 5.8 Celsius degrees over the period 1990 to 2100, and the sea level is projected to rise by 0.1 to 0.9 metres over the same period.
IPCC uses the best available predictions and their reports are under strong scientific scrutiny. The IPCC concedes that there is a need for better models and better scientific understanding of some climate phenomena, as well as the uncertainties involved. Critics point out that the available data is not sufficient to determine the real importance of greenhouse gases in climate change. Sensitivity of climate to greenhouse gases may be over (or under) estimated because of some flaws in the models and because the importance of some external factors may be misestimated.
On the other hand, predictions are based on scenarios, and the IPCC did not assign any probability to the 35 scenarios used.
The Kyoto Protocol to the UNFCCC proposes binding greenhouse gas limits for developed countries.
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